In gas turbine engines it is typically desirable to control the engine's thrust output. Changes in the engine's thrust output may occur for various reasons such as sudden transient decreases in thrust output corresponding to temporary decreases in turbine efficiency. For example, these decreases in efficiency may occur following periods of sudden acceleration in which differences in thermal expansion of the engine's components result in a brief period of excessive tip clearance between the engine's turbine and shroud. An engine's thrust output will also gradually decrease with time corresponding to gradual deterioration of components in the engine. Typically, an engine manufacturer will provide an adequate thrust margin which allows for these decreases in thrust and still insures that the engine will meet certain minimum thrust levels throughout the engine's life until overhaul. When an engine is operating at maximum thrust, the thrust margin is typically obtained by isothermally holding the engine's turbine temperature at a maximum level so as to provide necessary thrust while protecting engine components from excessive temperatures. By holding the turbine temperature isothermally a new engine is operated at a much higher temperature than necessary to provide the required thrust. However, when turbine temperature is held isothermally, thrust and fan operating line are not maintained constant as deterioration occurs. Therefore as the engine deteriorates eventually the actual isothermally held temperature will be required to obtain the minimum desired maximum thrust levels. Thus, throughout much of an engine's operating life the engine must be operated at temperatures in excess of that which is required to maintain desired thrust levels. Requiring operation at these increased temperatures results in more rapid deterioration of the engine, therefore requiring a greater frequency of overhauls and a greater expense of operation. An alternative to holding turbine temperature isothermally provides for the monitoring of engine pressure ratio. While these pressure ratio control systems do not require the engine temperature to be operated in excess of that which is required to maintain desired thrust, these systems do require the use of pressure sensors to be added to the engines. These sensors provide increased expense to the engine, in addition, these sensors and associated control result in additional maintainability and reliability concerns corresponding to adding additional components to the engine.